1. Field of the Invention
The present invention relates to a control device for the formation of several simultaneous radar reception beams in an electronic scanning antenna. It can be applied especially to the control of the radiation pattern of an electronic scanning antenna with a view to reconfiguring the reception beams with high flexibility, whatever the passband of the radar.
2. Description of the Prior Art
An electronic scanning antenna comprises a plurality of radiating elements carrying out both the transmission and the reception of a microwave signal. A transmission or reception beam is constituted by all the signals sent or received by each element. To orient a beam in a given direction xcex8, time delays have to be created between signals sent or received by the different radiating elements. To obtain a similar effect, there are known ways of creating a phase delay between these signals. The phase shift xcfx861xe2x88x92xcfx862 between the signals sent or received by the two radiating elements is therefore given by the following relationship:                                           Φ            1                    -                      Φ            2                          =                                            d              ⁢                              xe2x80x83                            ⁢              sin              ⁢                              xe2x80x83                            ⁢              θ                        c                    xc3x97          2          ⁢          π          ⁢                      xe2x80x83                    ⁢          f                                    (        1        )            
where d, f and c respectively represent the distance between the two radiating elements, f the frequency of the signals and c the speed of light, the time delay created being             T      1        -          T      2        =                    d        ⁢                  xe2x80x83                ⁢        sin        ⁢                  xe2x80x83                ⁢        θ            c        .  
For its part, the phase shift xcfx861xe2x88x92xcfx862 is equal to 2xcfx80f(T1xe2x88x92T2).
Instead of the approach described here above, which makes use of microwave control circuits, it may be preferred to adopt an approach using optical control circuits, especially for passband problems. The above relationship (1) indeed shows a drawback, in that the phase shift depends on the frequency. As a consequence, if the frequency varies, the pointing angle varies too. This method of orientation of a beam is therefore not suited for a wideband radar. However, it is not possible, with microwave techniques, to create a temporal delay between the signals except by creating the above phase shift, unless it is chosen to implement a device that is prohibitively costly and bulky.
The use of optical techniques does away with the above-mentioned drawback, by controlling the radiating elements directly through time delays, without using the contrived solution of phase shifts, these delays being created in the optical domain. To this end, solutions for the optical control of electronic scanning antennas have already been implemented. For transmission, many optical control architectures have therefore already been proposed in order to check the radiation pattern at transmission.
With regard to the reception of the signals by the antenna, beam formation calls for a very great dynamic range which is as yet beyond the reach of optical components. The dynamic range, in terms of radar, is characterized by the signal-to-noise ratio where the term xe2x80x9cnoisexe2x80x9d includes the intermodulation phenomena caused by the non-linearities of the chain generally known in the literature as the SFDR or xe2x80x9cSpurious Free Dynamic Rangexe2x80x9d.
The speed with which the beam is switched over in a given direction by means of a command is another difficulty, which is a second-order difficulty with respect to the dynamic range.
In order to overcome this problem of dynamic range, an optical control architecture, based on correlation, was presented in the French patent application No. 94 11498 and then complemented by an architecture presented in the French patent application No. 98 07240. This optical architecture with correlation can be used, in both reception and transmission, as a system of control based on temporal delays along the two planes in elevation and in relative bearing. However, this architecture can be used for the formation of only one beam. It does not provide for a multiple-beam reception, namely reception with many simultaneous beams. Now, for many radar applications, it is necessary, at least in one of the planes of the radar, elevation or relative bearing, to form several beams at reception, for example sets of sum and difference beams.
If the passband is not essential for certain radar applications that may accept medium bandwidths, then multiple-beam reception is possible in control techniques based on microwave circuits alone. At reception, the radar echo is detected on a array antenna by a matrix of n rows by m columns of microwave detectors constituting the antenna block. These elementary signals are individually weighted in amplitude and in phase and then summed to form a reception beam. This beam is characterized by its angular direction with respect to the normal to the antenna and by its radiation pattern. In order to simultaneously form several reception beams, it is necessary to divide the elementary signals to direct them towards different weighting matrices and different summators. These weighting operations are performed in microwave technology, and are unchangeable. The reconfiguring of the reception beams is nevertheless possible through the use of architectures for beam-formation by computation known as FFC architectures. These architectures are nevertheless expressed by an increased complexity in terms of radar treatment. These are indeed real-time processing operations that dictate the use of many complex and costly processors. In other words, the complexity of the processing limits the number of receivers in the FFC architectures.
Thus, it is not possible to carry out a simple implementation of a multiple-beam radar with a wide dynamic range, whether by the use of a microwave command, which does not permit a dynamic allocation of the beams formed, or in the digital domain which dictates a limitation on the number of channels (sub-arrays) sampled.
An aim of the invention especially is to provide for a simple embodiment of a multiple-beam reception with a wide dynamic range.
To this end, an object of the invention is a control device for the formation of radar reception beams of an electronic scanning antenna comprising a array of detectors of microwave signals positioned in n sub-arrays of detectors, the control device comprising:
means for making, in the microwave domain, a first partial combination of the signals received, this combination being obtained according to each sub-array;
optical means to make a second partial combination in the optical domain, the optical means comprising at least n optical sources each producing an optical signal modulated at the frequency of the reception microwave signals f by an associated microwave combination signal, means for the division of this optical signal into r optical signals, means for the optical phase-shifting of each of the nxr optical signals obtained, r means of combining each group of n optical signals and r means of detection of the modulation signal f assigned to each group of n optical signals to form r microwave reception beams.
The main advantages of the invention, moreover, are that it reduces the complexity of the digital processing in relation to the formation of FFC radar beams, procures immunity against electromagnetic disturbances, provides a gain in lightness and compactness, and can be applied to all radar frequency bands.